Liquid-phase column chromatography is one of the oldest chromatographic methods, but until these last few years it had not been widely used, mainly because of the slowness of the separations, the lack of a choice of stationary phases, and the lack of detectors. Now that these obstacles have been overcome, there is a resurgence of this method in a more modern form, under high pressure.
Since liquid-phase chromatography can be subdivided into several classes on the basis of the nature of the phases or of the phenomena involved in the separation of the components, it may be well to specify here that the invention is concerned more particularly with partition chromatography using a grafted stationary phase. In this technique, the separation of the species is based on the difference in their interaction with the molecules with different functional groups which have been grafted onto an appropriate solid, typically a silica gel.
In such a chromatographic column, a mobile liquid phase, the so-called eluting phase, moves through the stationary phase formed by grafting different organic molecules onto a silica gel. These organic molecules will hereinafter be called grafts.
The differential retention by these grafts of the chemical species contained in the mobile phase makes it possible to separate them.
The principal grafts are:
Amines: PA0 Nitriles: PA0 Alkyl chains: PA0 Diols:
--(CH.sub.2).sub.3 --NH.sub.2, PA1 --(CH.sub.2).sub.3 --N(CH.sub.3).sub.2, --(CH.sub.2).sub.3 --NH--(CH.sub.2).sub.2 --CH.sub.3, ##STR1## Diamines: --(CH.sub.2).sub.3 --NH(CH.sub.2).sub.2 --NH.sub.2 PA1 --(CH.sub.2).sub.n --CN PA1 --(CH.sub.2).sub.n --CH.sub.3 PA1 --(CH.sub.2).sub.3 --O--CH.sub.2 --CH OH--CH.sub.2 OH
and so forth.
The grafting process usually consists of a condensation reaction between the silanol groups (.tbd.Si--OH) of the silica gels and a chlorosilane or an alkoxysilane in accordance with the following reaction: ##STR2## where X=Cl (chlorosilane) or X=OCH.sub.3, OC.sub.2 H.sub.5 (alkoxysilane).
Partition chromatography, whose potential uses have considerably increased as a result of the utilization of diverse grafts, is suitable particularly for the separation of homolog species or of species comprising groups of a different nature, polar or nonpolar (for which conventional chromatography and reversed-phase partition chromatography, respectively, are used). It offers the advantage of permitting the analysis of compounds which are too strongly adsorbed on virgin silica gels. (Adsorption chromatography.) The presence and choice of grafts therefore are the essential and determinant characteristics of the method concerned.
Since applicants' objective is primarily the separation of the components (or families of components) of a crude petroleum, or of a fraction thereof, or of the products resulting from the refining thereof, the solubility of the components to be separated in a hydrocarbon (such as n-hexane) inclines it toward a partition chromatography where the stationary phase is a silica grafted with a functional group of the amine type, for example, an aminopropyl, and where the mobile phase is a nonpolar solvent such as n-hexane.
Using such a known chromatographic column, the solutes are roughly eluted in the sequence of their polarity, with the more polar components emerging from the column last. The components of the purely aromatic type thus are retained longer than those of the olefinic or saturated type.
However, in the case of a crude petroleum, and more particularly of a so-called heavy petroleum, a sizable proportion of the components is formed by aromatic rings substituted to a greater or lesser extent with long alkyl chains, and the behavior of these complex molecules toward the nonpolar eluant (n-hexane) or the stationary phase is intermediate between that of the components of the olefinic or saturated type and the components of the purely aromatic type. Their proper separation therefore is rendered extremely difficult.